Aircraft flat panel displays presently under development have extremely high theoretical thermal stresses. Presently known back light dimmers require as much as 10 watts to provide proper luminance for an aircraft environment. Ten watts is nearly half of a typical total display unit power demand. Therefore, any significant decrease in the backlight power requirements would also significantly reduce the display unit thermal stress. Assignee's co-pending application Ser. No. 07/280,482, filed Dec. 6, 1988 entitled "Fluorescent Lamp Dimmer" teaches a fluorescent lamp dimming system using high frequency pulsating AC with two independent power control variables for dimming, namely pulse width and pulse frequency. As taught in Ser. No. 07/280,482, multiple lamps are driven in order to provide single failure redundancy. Such an approach requires excessive power, but helps to create some redundancy in order to avoid catastrophic failure such as a dark display. The teachings of Ser. No. 07/280,482 are incorporated herein by this reference in their entirety. Since such a multiple lamp approach requires at least two fluorescent lamps in each display, if one lamp fails, the other lamp will provide some luminance for a useable display. Since an AC lamp drive is used, power must be applied to the heater electrodes at each end of each lamp for a total of four heaters. The heater power produces no useable light. In addition, the power lost in the cathode fall in each lamp provides no light. Therefore, while the Ser. No. 07/280,482 application has certain advantages over the prior art, certain of its features are inefficient when compared to the invention disclosed herein.
For example, matching luminance between multiple lamps over the complete dimming range and over a wide ambient temperature range is very difficult to achieve with the system as disclosed in Ser. No. 07/280,482. A luminance mismatch between lamps or along a lamp creates a luminance nonuniformity over the surface of the display. Further, lamp cathodes used in such an AC system were made small in order to conserve power. Unfortunately, this contributes to a very short lamp life. The new apparatus disclosed herein consumes significantly less power than the AC system and does not require matching luminance.
The present invention provides a fluorescent lamp dimmer which drives only one cathode at a time with pulsed DC energy. The pulsed DC drive energy is switched to the other cathode before any significant mercury migration can take place within the lamp. Other DC drive techniques inherently have problems with mercury migration because they do not alternate drive currents from one cathode to the other so as to avoid mercury migration. Other DC lamp drives only heat one cathode, but after about 30 minutes, depending upon lamp size and lamp temperature, a mercury migration occurs inside the fluorescent lamp that causes a significant luminance variation along the lamp. It may also cause lamp ignition problems when the lamp is required to be very dim. In addition, a change in lamp color from white to pink along the lamp may occur due to lack of local mercury vapor pressure within a DC driven lamp. The present invention allows significant power savings for the same light output, provides cathode redundancy with a single more efficient lamp, and solves the mercury migration problem of other DC drive techniques.
The invention is particularly useful for flat panel aircraft displays which present a two-fold problem. The first problem requires finding a solution for reducing power while maintaining the same luminance flux. The second problem relates to maintaining redundancy so that a single lamp failure will not be catastrophic and result in an unusable display. With the DC lamp driver discussed above, only one end or filament of a lamp is emitting electrons. Therefore, only the emitting end must be heated to thermionic emission temperature with filament heater power. When using an AC drive, the arc current will alternate in direction at a 60 Hz to 16 KHz rate. Since the thermal time constant of the filament heater is relatively long (i.e., several seconds, compared to the switching periods) the AC system must simultaneously heat both filaments to thermionic emission temperature. Therefore, both filaments are behaving as cathodes and both cathodes are required for the lamp to operate normally.
It is also desirable to use only one longer lamp instead of two lamps to further reduce power loss by limiting the loss to only one cathode fall instead of the usual two. Until the present invention, redundancy for reliability required two lamps. A major failure mechanism in a fluorescent lamp of the type used in flat panel displays is cathode failure. If a single lamp were used with either of the AC or DC drive systems described above, and a single cathode were to fail, the lamp would be catastrophically dark in the DC drive case and dim and flicker badly in the AC drive case.
The fluorescent lamp dimmer as provided in accordance with the present invention solves these problems by allowing the use of one longer lamp while driving and heating only one cathode at a time. The drive is switched to the other cathode before mercury migration can take place. Typically, mercury migration takes place in about 30 minutes. If a cathode failure is detected, the switching done in accordance with the present invention will not occur, thus, providing an immunity to a single cathode failure resulting in a catastrophic failure. Instead, the lamp will dim normally with the single failure and without flicker. Some luminance variation due to mercury migration will occur until the failed lamp can be replaced, but the display will be usable. In addition, very significant power savings are achieved by apparatus provided in accordance with the present invention because instead of the heating loss in four cathodes and the power loss in two cathode falls, the apparatus of the invention can drive a single longer lamp and produce the same luminance flux from the positive column arc while only requiring one filament to be heated. Thus, power loss in only one cathode fall is experienced.
In one particular example of the types of lamps being used for an aircraft flat panel display, each filament heater requires one watt and the power loss in the dark cathode fall region is about 0.75 watts. Thus, if an AC or DC system other than the present invention is used which requires two lamps for a single failure reliability, the power required for driving the lamps, excluding the light producing positive column arc power totals as follows:
______________________________________ Description = Watts ______________________________________ Four filament heaters at one watt = 4.0 watts Two cathode falls at 0.75 watts each = 1.5 watts Total = 5.5 watts ______________________________________
This power produces no light. Light output only comes from the positive column arc power of 4.5 watts which is the same for the present invention as the other AC and DC techniques described above. For the new technique, the power required to drive the lamp totals as follows.
______________________________________ 1 Filament Heater = 1.0 Watts 1 Cathode Fall = 0.75 Watts Total 1.75 Watts ______________________________________
This power produces no light, but is 3.75 watts lower than the other techniques. Thus, the present invention, as used in this example, would save 3.75 watts out of a total of 10 watts as originally required.